1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive measuring a writer/reader gap in a disk drive by measuring write/read times relative to a sync mark.
2. Description of the Prior Art
When manufacturing a disk drive, servo sectors 20–2N are written to a disk 4 which define a plurality of radially-spaced, concentric data tracks 6 as shown in the prior art disk format of FIG. 1. Each data track 6 is partitioned into a plurality of data sectors wherein the servo sectors 20–2N are considered “embedded” in the data sectors. Each servo sector (e.g., servo sector 24) comprises a preamble 8 for synchronizing gain control and timing recovery, a sync mark 10 for synchronizing to a data field 12 comprising coarse head positioning information such as a track number, and servo bursts 14 which provide fine head positioning information. The coarse head position information is processed to position a head 16 over a target track during a seek operation, and the servo bursts 14 are processed to maintain the head 16 over a centerline of the target track while writing or reading data during a tracking operation.
The head 16 is coupled to a distal end of an actuator arm 18 which is rotated about a pivot 20 by a voice coil motor (VCM) formed from a voice coil 22 coupled to the base end of the actuator arm 18 and permanent magnets (not shown). When current is applied to the voice coil 22, the resulting magnetic flux interacts with the magnet flux of the permanent magnets to generate a rotational torque thereby rotating the actuator arm 18 about the pivot 20 to position the head 16 over a target data track.
With certain heads, such as magnetoresistive (MR) heads, the writer is separated from the reader resulting in a writer/reader gap. This is illustrated in FIG. 2 which shows a head 16 comprising a writer 24 that is spatially separated along the length of a data track from a reader 26 forming a writer/reader gap 28. FIG. 2 also shows a prior art format of a data track including the servo bursts 14 (A, B, C, D) at the end of a servo sector, followed by a DC gap 30 preceding a data sector comprising a preamble 32, sync mark 34 and user data 36. In the embodiment of FIG. 2, the disk is rotating such that the data track moves under the head 16 from right to left which is the same as the head 16 moving from left to right over the data track. Thus, in this configuration the reader 26 is “in front” of the writer 24 such that the writer 24 “lags” the reader 26 by the writer/reader gap 28 as the data track passes under the head 16.
The writer/reader gap 28 varies from disk drive to disk drive due to various manufacturing tolerances. The length of the DC gap 30 is therefore selected to be long enough to ensure the servo bursts 14 are not overwritten when the preamble 32 of the following data sector is written. The length of the DC gap 30 is selected to account for the worst case (longest) writer/reader gap 28 (plus some margin). However, a long DC gap 30 reduces the format efficiency by consuming disk space that could otherwise be used for storing user data.
The writer/reader gap 28 also creates uncertainty as to when to assert the read gate (read gate delay) to enable timing recovery to begin synchronizing to the preamble 32 of the data sector. Asserting the read gate too early causes timing errors due to timing recovery attempting to synchronize to the spurious data in DC gap 30. Asserting the read gate too late (too far into the preamble 32) may prevent timing recovery from fully synchronizing before encountering the sync mark 34. This problem is exacerbated by the varying length of writer/reader gap 28 along the length of the data track over the stroke of the actuator arm 18 (radial location of the head 16 from inner diameter to outer diameter) as illustrated in FIG. 3.
U.S. patent application Ser. No. 10/016,342 entitled “DISK DRIVE WITH OPTIMIZED READ GATE DELAY” discloses a technique for adjusting the read gate delay until the lowest bit error rate is achieved when reading a data sector following a servo sector. This technique, however, provides only a rough estimate of the writer/reader gap 28 since other factors also affect the bit error rate. Further, this technique only optimizes the read gate delay and does not optimize the length of the DC gap 30 (write gate delay) in order to increase format efficiency. Still further, relying on bit error rate measurements may take several revolutions (several retries) to obtain enough data for a reliable estimate. This can increase the manufacturing time since the optimal read gate delay must be estimated for multiple radial locations of the head 16 (e.g., for each zone on the disk) to account for the changes in the writer/reader gap 28 over the stroke of the actuator arm 18.
There is, therefore, a need to quickly and accurately measure the writer/reader gap in a disk drive in order, for example, to optimize the read gate delay and/or the write gate delay.